Generally a hearing aid system according to the invention is understood as meaning any system which provides an output signal that can be perceived as an auditory signal by a user or contributes to providing such an output signal, and which has means adapted to compensate for an individual hearing loss of the user or contribute to compensating for the hearing loss of the user. These systems may comprise hearing aids that can be worn on the body or on the head, in particular on or in the ear, or that can be fully or partially implanted. However, a device whose main aim is not to compensate for a hearing loss, for example a consumer electronic device (televisions, hi-fi systems, mobile phones, MP3 players etc.), may also be considered a hearing aid system, provided it has measures for compensating for an individual hearing loss.
Within the present context a hearing aid can be understood as a small, battery-powered, microelectronic device designed to be worn behind or in the human ear by a hearing-impaired user. Prior to use, the hearing aid is adjusted by a hearing aid fitter according to a prescription. The prescription is based on a hearing test, resulting in a so-called audiogram, of the performance of the hearing-impaired user's unaided hearing. The prescription is developed to reach a setting where the hearing aid will alleviate a hearing loss by amplifying sound at frequencies in those parts of the audible frequency range where the user suffers a hearing deficit. A hearing aid comprises one or more microphones, a battery, a microelectronic circuit comprising a signal processor, and an acoustic output transducer. The signal processor is preferably a digital signal processor. The hearing aid is enclosed in a casing suitable for fitting behind or in a human ear.
Within the present context a hearing aid system may comprise a single hearing aid (a so called monaural hearing aid system) or comprise two hearing aids, one for each ear of the hearing aid user (a so called binaural hearing aid system). Furthermore the hearing aid system may comprise an external device, e.g. a smart phone, having software applications adapted to interact with other devices of the hearing aid system. Thus within the present context the term “hearing aid system device” may denote a hearing aid or an external device.
In a traditional hearing aid fitting, the hearing aid user travels to an office of a hearing aid fitter, and the user's hearing aids are adjusted using the fitting equipment that the hearing aid fitter has in his office. Typically the fitting equipment comprises a computer capable of executing the relevant hearing aid programming software and a programming device adapted to provide a link between the computer and the hearing aid.
Hearing loss of a hearing impaired person is quite often frequency-dependent and may not be the same for both ears. This means that the hearing loss of the person varies depending on the frequency. Therefore, when compensating for hearing losses, it can be advantageous to utilize frequency-dependent amplification. Hearing aids therefore often provide band split filters in order to split an input sound signal received by an input transducer of the hearing aid, into various frequency intervals, also called frequency bands, which are independently processed. In this way it is possible to adjust the input sound signal of each frequency band individually to account for the hearing loss in respective frequency bands. The frequency dependent adjustment is normally done by implementing a band split filter and a compressor for each of the frequency bands, hereby forming so-called band split compressors, which may be combined to form a multi-band compressor. In this way it is possible to adjust the gain individually in each frequency band depending on the hearing loss as well as the input level of the input sound signal in a respective frequency band. For example, a band split compressor may provide a higher gain for a soft sound than for a loud sound in each frequency band.
Traditionally a hearing aid system is fitted based only on the recorded audiogram for the individual haring aid system user. However, it is well known that the benefit of wearing a hearing aid system may differ significantly for users having similar or even identical audiograms.
Therefore, there is a need to improve the audiological fitting of hearing aid systems. U.S. Pat. No. 7,804,973 B2 discloses a method of selecting parameters for one or more noise reduction algorithms based on the individual user's SNR loss. The term SNR loss is defined as the average increase in signal-to-noise ratio (SNR) needed for a hearing impaired patient relative to a normal hearing person in order to achieve similar performance (50% word recognition) on a hearing in noise test, at levels above the hearing threshold. According to an aspect of the disclosed method a degree of restoration/improvement of the SNR of noise contaminated input signals of the hearing aid system has been made dependent on the SNR loss of the individual user. However, this method does not use a classification of the type of hearing loss to guide the selection of hearing aid features, parameter settings, and gain rationales that have been specifically adapted for each type of hearing loss to be most beneficial in addressing the SNR loss.
The paper “A signal-to-noise ratio model for the speech-reception threshold of the hearing impaired” by Plomp published in the Journal of Speech and Hearing Research, Vol. 29, 146-154, June 1986 discloses a preferred method for measuring a Speech-Reception-Threshold (SRT) based on an adaptive trial-by-trial adjustment of the sound pressure level of a number of carefully selected sentences. The SRT is found as the sound pressure level required for obtaining a speech intelligibility of 50%. The paper further states that whereas word lists may have priority for diagnostic purposes, short meaningful sentences are more representative of conversational speech so that the threshold conditions are identical to the critical situations in normal practice. Sentences have the additional advantage that the slope of the psychometric function representing the intelligibility score as a function of sound-pressure level is steeper (20%/dB) than for single words. This is beneficial to an accurate estimation of the SRT.
The paper also defines speech communication handicap as elevation of the speech reception threshold (SRT) over that of the average SRT for individuals with normal hearing. There are two factors that can cause the SRT to be elevated, audibility loss (the functional hearing deficit that predominantly makes at least a part of the speech spectrum inaudible), and distortion loss (the functional hearing deficit that is due to distorted auditory processing). Audibility loss represents a loss of sensitivity, while distortion loss is the reduced ability to understand speech in background noise when both the speech and noise are audible. The SRT in quiet is elevated by both audibility loss and distortion loss, and the SRT in supra-threshold noise is elevated only by distortion loss. Thus, an individual's speech communication handicap can be characterized with two SRTs, one in quiet and the other in supra-threshold noise. While this is useful information for classifying functional impairment caused by hearing loss the article does not provide an automatic, effective and precise method of quantifying the extent of this impairment.
The paper “On the auditory and cognitive functions that may explain an individual's elevation of the speech reception threshold in noise” by Houtgast and Festen published in International Journal of Audiology 2008; 47: 287-295, considers a variety of auditory and cognitive functions that may underlie the so called distortion, that represents the additional factor that has to be taken into account in order to understand why a pure-tone audiogram is not sufficient to explain the varying results of speech-in-noise tests obtained by hearing aid users having similar audiograms.
Further the paper discloses a calculation of the Speech Intelligibility Index (SII) at a given SRT, noting that this calculation takes into consideration frequency-specific thresholds of audibility. It was found that when the SRT is elevated due only to the effects of impaired audibility, which is considered in the SII calculations, the SII at the elevated SRT remains the same as that of normally hearing individuals. However, if elevation of the SRT is due to the effects of increased distortion, the SII at the SRT is increased over that of normally hearing individuals.
Thus the paper discloses how measurements of the SRT and pure-tone thresholds, together with SII calculations, can be used to characterize the cause of communication handicap as due primarily to impaired audibility or distortion. While this is useful information for classifying functional impairment caused by hearing loss the article does not provide an automatic, effective and precise method of quantifying the extent of this impairment.
It is therefore a feature of the present invention to provide an improved method of fitting a hearing aid system.
It is another feature of the present invention to provide a hearing aid fitting system adapted to carry out an improved method of fitting a hearing aid system.